Final answer:
Direct-acting agonists have modified groups that allow them to mimic neurotransmitters and bind to receptors to elicit similar biological responses.
Step-by-step explanation:
Direct-acting agonists typically have a modified group that mimics the structure of a neurotransmitter, allowing them to bind to a receptor and produce a similar effect to the endogenous ligand. For instance, modifications might include the replacement of a carboxyl group with a benzene ring, as seen in norepinephrine derivatives which are designed to interact with adrenergic receptors such as alpha (α)-adrenergic and beta (β)-adrenergic receptors. These structures may possess hydrophilic and hydrophobic domains linked either by ester or amide bonds, with ester linkages being particularly important due to their susceptibility to hydrolysis, leading to metabolic degradation and inactivation of the compound.